1. Field of the Invention
The present invention relates to the field of the production of wafers or multilayer semiconductor substrates (also called “multilayer semiconductor wafers”) effected by the transfer of at least one layer formed from an initial substrate onto a final substrate where the layer transferred corresponds to a portion of the initial substrate. The layer transferred may also include all or part of a component or a number of microcomponents.
2. Description of Related Art
More specifically, this invention relates to the problem of heterogeneous distortions that appear when transferring a layer from a substrate called the “donor substrate” onto a final substrate called the “receiver substrate”. Such distortions have been observed in the case of three-dimensional integration technology of components (3D integration) which requires the transfer of one or more layers of microcomponents onto a final substrate but also in the case of the transfer of circuits as well as in the manufacture of backside-illuminated imagers. The layer(s) transferred include(s) microcomponents (electronic, optoelectronic, etc.) produced at least in part on an initial substrate. These layers are then stacked onto a final substrate which may itself include additional components. Especially because of the very small size and the large number of microcomponents on a given layer, each layer transferred must be positioned on the final substrate with considerable precision in order to observe very accurate alignment with the underlying layer. In addition, it may be necessary to carry out processing on a layer after its transfer, for example, to form other microcomponents, to bring microcomponents to the surface, to make interconnections, and the like.
However, applicant has found that after the transfer, there are cases where it is very difficult, if not impossible, to form additional microcomponents in alignment with microcomponents formed before the transfer. This phenomenon of misalignment is described in reference to FIGS. 1A to 1E which show an example of the production a three-dimensional structure including the transfer onto a final substrate, a layer of microcomponents formed on an initial substrate and the formation of an additional layer of microcomponents on the exposed surface of the initial substrate after bonding. FIGS. 1A and 1B show an initial substrate 10 on which is formed a first series of microcomponents 11. The microcomponents 11 are formed by photolithography using a mask to define the areas of formation of patterns corresponding to the microcomponents 11 to be produced.
As shown in FIG. 1C, the face of the initial substrate 10 including the microcomponents 11 is then brought into direct contact with one face of a final substrate 20. The bonding between the initial substrate 10 and the final substrate 20 is usually made by molecular adhesion. One thus obtains a buried layer of microcomponents 11 at the bonding interface between substrates 10 and 20. After bonding and as shown in FIG. 10, the initial substrate 10 is thinned down to remove a portion of the material lying above the layer of microcomponents 11. One then obtains a composite structure 30 consisting of the final substrate 20 and a layer 10a corresponding to the remaining portion of the initial substrate 10.
As shown in FIG. 1E, the next step in the production of the three-dimensional structure is to form a second layer of microcomponents 12 on the exposed surface of the thinned-down initial substrate 10, or to carry out additional technological processes on the exposed surface, aligned with the components included in the layer 10a (making contacts, interconnections, etc.). For simplicity, we will use the term “microcomponents” in the following text to refer to the devices formed as a result of the technological processes undertaken in or on the layers, and the positioning of which must be controlled with precision. These components may thus be active or passive, merely making contact or representing interconnections.
Thus, in order to form the microcomponents 12 in alignment with the buried microcomponents 11, one uses a photolithographic mask similar to the one used to form the microcomponents 11. The layers transferred, such as the layer 10a, typically include marks with respect to both the microcomponents and the wafer forming the layer and which are used especially by the tools for positioning and alignment during the stages of technological processing such as those implemented during photolithography.
However, even when using positioning tools, misalignment may occur between some of the microcomponents 11 and 12 resulting in misalignments Δ11, Δ22, Δ33, Δ44, shown in FIG. 1E (corresponding to the observed discrepancies between the pairs of microcomponents 111/121,112/122, 113/123 and 114/124). These misalignments are due to heterogeneous distortions which appear in the layer coming from the initial substrate during its assembly with the final substrate. The distortions result in the displacement of some microcomponents 11. In addition, some microcomponents 12 formed on the exposed surface of the substrate after transfer, show variations in position with respect to the microcomponents 11 which may be in the order of several hundred nanometers or even microns.
This phenomenon of misalignment (also called “overlay”) between the two layers of microcomponents 11 and 12 may be a source of short circuits, distortions in the stacking, or faulty connection between the microcomponents of the two layers. This phenomenon of misalignment thus leads to a reduction in the quality and the value of the multilayer semiconductor wafers manufactured. The impact of this phenomenon is becoming increasingly critical due to the steadily increasing requirements with respect to the miniaturization of microcomponents and their integration density per layer.
The alignment problems in the manufacture of three-dimensional structures are well known. The document Burns et al. titled “LiA Wafer—Scale 3-D Circuit Technology Integration”, IEEE Transactions On Electron Devices, vol. 53, No. 10, October 2006, describes a method to detect variations in alignment between bonded substrates. The document Haisma, et al. “Silicon-Wafer Fabrication And (Potential) Applications Of Direct-Bonded Silicon”, Philips Journal of Research, Vol. 49, No. V2, 1995, stresses the importance of the flatness of the plates especially during the polishing steps in order to obtain good quality final wafers, i.e. with the least possible distortions between microcomponents. It is not desirable to introduce distortions in a heterogeneous substrate during its transfer onto another substrate.
The present invention provides a solution to these problems.